Classifying cancer-causing substances

Various international agencies follow a scientific process and consider the weight of scientific evidence to determine whether a substance or chemical causes cancer. They have different ways of describing how clear and strong the scientific evidence is, and how confident scientists are about the risk associated with exposure.

The NTP in the United States follows a similar process to describe whether a substance causes cancer in humans. Following the review of animal and human scientific information, NTP scientists classify substances as either: